1. Field of the Invention
The present invention relates to organic light emitting devices such as organic electroluminescence devices (hereinafter referred to as organic EL devices) and, particularly, to an organic light emitting device which emits a spectral component in the wavelength range of blue or a spectral component in the range of a wavelength shorter than that of blue, and is capable of obtaining white light emission or blue light emission.
2. Description of the Background Art
In recent years, requirements are increasing for thin display devices having a thickness of approximately several mm and capable of providing full-color display with the advance of information technology (IT). As such thin display devices, organic EL devices have been developed.
Three methods are roughly proposed as means for realizing full-color display. The first method is such that a large number of light emitting devices are arranged which emit respective monochromatic lights of red, green and blue, which are three primary colors of light. The second method is to adopt a combination of a white light emitting diode and a color filter which allows transmission of monochromatic lights of three primary colors of light. The third method is to adopt a light emitting diode which emits ultraviolet light or blue light and wavelength converting means for wavelength-converting the light from the light emitting diode into monochromatic lights of three primary colors of light. It is uncertain at present which method is superior to others.
In general, while it is comparatively easy to realize highly efficient organic light emitting devices which emit green light, it is difficult to realize those which emit red light. It is more difficult to realize organic light emitting devices which emit blue light and white light.
Requirements are now made for light emitting devices emitting blue light and white light in each of the above mentioned methods for achieving full-color display as described above. Thus, a demand is increasing for achieving an organic light emitting device which emits a spectral component in the wavelength range of blue or a spectral component in the wavelength range of a shorter wavelength than that of blue, and is capable of obtaining blue light or white light, by employing any one of the above described methods.
For example, Forrest, Stephen R. et al. discloses in Appl. Phys. Lett., 1999, 75(1), pp. 4–6 an organic EL device emitting green light which has a mixture luminescent layer made by mixing fac-tris(2-phenylpyridine)iridium (hereinafter abbreviated as Ir(ppy)) being a green phosphorescence emitting substance into 4,4′-bis(carbazol-9-yl)-biphenyl (hereinafter abbreviated as CBP) at a concentration of 1 to 12 mass %.
The document describes that a maximum luminous efficiency was attained in the organic EL device when the concentration of Ir(ppy) in the mixture luminescent layer was 6 mass %, resulting in such properties as a luminescence peak wavelength being 510 nm, a full-width at half maximum value (FWHM value) of a luminescence peak being 70 nm, and a chromaticity coordinate of Commission International d'Eclairage (CIE) being (x=0.27, y=0.63).
The document mentions that a blue emitting component is negligible which is considered to be provided from a singlet excited state of CBP even though a current density is raised to 100 mA/cm2, and thus, complete energy transfer occurs from CBP to Ir(ppy) in this organic EL device (Refer to FIG. 4 in the document). It is thus impossible to obtain blue light emission or white light emission in the organic EL device by employing the technology disclosed in the above described document.
A simplified molecular formula for CBP is represented by C36H24N2 where one molecule includes 36 carbon atoms, 2 nitrogen atoms and 24 hydrogen atoms. A molar mass of CBP is 484.60 g/mol and its melting point is 282° C. to 283° C. The wavelength of an absorption edge on the side of the longest wavelength in an optical absorption spectrum in a visible light range of CBP is 400 nm. A structural formula for CBP is represented by a chemical formula (1) shown below.

Moreover, Forrest, Stephen R. et al. disclose in Appl. Phys. Lett., 1999, 74(3), pp. 442–444 an organic EL device emitting red light which has a mixture luminescent layer made by mixing 2,3,7,8,12,13,17,18-octaethenyl-21H,23H-porphine platinum (II) (hereinafter abbreviated as PtOEP) being a red phosphorescence emitting substance into CBP at a concentration of 6 mass %.
In the disclosed organic EL device, a red emitting component with a 650 nm luminescence peak wavelength, which is considered to be provided from a triplet excited state of PtOEP, covers most of the photons discharged from the device, while a red emitting component with a 580 nm luminescence peak wavelength, which is considered to be provided from the singlet excited state of PtOEP, is not detected. In this organic EL device, however, any blue emitting component is not detected, which is considered to be provided from the singlet excited state of CBP (Refer to FIG. 3 in the document). It is thus impossible to obtain blue light emission or white light emission in the organic EL device by employing such technology as disclosed in the described document.
In addition, Forrest, Stephen R. et al. disclose in Appl. Phys., 2000, 87(11), pp. 8049–8055 an organic EL device having a mixture luminescent layer made by mixing tris(thenoyltrifluoroacetone)(1,10-phenanthroline)europium (hereinafter referred to as Eu(tta)3phen), which is a compound similar to europium (III)-phenanthroline-tri-thenoyltrifluoro-acetylacetonate (hereinafter abbreviated as Eu(ttfa)3phentp) emitting red light, into CBP to a concentration of 1 mass %.
In the disclosed organic EL device, a red light emission with a 612 nm luminescence peak wavelength is obtained which is considered to be provided from the triplet excited state of Eu(tta)3phen. When a current density is 100 mA/cm2, in this organic EL device, a 505 cd/m2 luminance is attained, and a 0.22% external quantum efficiency and a 0.505 cd/A current luminous efficiency are obtained.
FIG. 1 in the described document shows respective energy levels of constituent materials. In this case, the energy level of Eu(tta)3phen is unclear; however, it is described that the level of a lowest unoccupied molecular orbital (LUMO) of CBP is 3.2 eV and the level of a highest occupied molecular orbital (HOMO) is 6.3 eV. It is thus evaluated that an energy gap between LUMO and HOMO is 3.1 eV. An ionization potential of CBP is considered to be 6.3 eV equal to the level of HOMO.
The organic EL device disclosed here is, however, the red light emitting device as described above. Therefore, it is impossible to obtain blue light emission or white light emission in the organic EL device by employing such technology as disclosed in the described document.
Moreover, Forrest, Stephen R. et al. disclose in Nature, 2000, 403, 6771, pp. 750–753 an organic EL device fabricated by forming a first type of mixture luminescent layer which is made by mixing 2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene] propane-dinitrile (hereinafter abbreviated as DCM2) being a red fluorescence emitting substance into CBP at a concentration of 1 mass %; forming a second type of mixture luminescent layer which is made by mixing the above described Ir(ppy) being a green phosphorescence emitting substance into CBP at a concentration of 10 mass %; and forming a stacked mixture luminescent layer which is made by stacking five pairs of the first and second types of mixture luminescent layers (i.e., 10 layers in total), with each pair including the first type and the second type of mixture luminescent layers.
The above described organic EL device exhibits such results that a red emitting component with a 590 nm luminescence peak wavelength which is considered to be produced from the singlet excited state of DCM2 covers 80% of the photons discharged from the device, and that a green emitting component with a 500 nm luminescence peak wavelength which is considered to be produced from the triplet excited state of Ir(ppy) covers the remaining approximately 20% of the photons. As described above, in this case, a blue emitting component with a 400 nm luminescence peak wavelength which is considered to be produced from the singlet excited state of CBP is hardly detected (Refer to FIG. 3 in the document). It is thus impossible to obtain blue light emission or white light emission in the organic EL device by employing the technology disclosed in this document.
A simplified molecular formula for DCM2 is C23H21ON3, and its molar mass is 355.43 g/mol. A structural formula for DCM2 is represented by a formula (11) shown below.

On the other hand, Japanese Patent Laid Open No. 8-157815 discloses an organic EL device emitting white light in which a thin film constituted by a copolymer of a coumarin derivative with a specified structure and N-vinylcarbazole, and fluorescent dye is formed as a luminescent layer. In this case, the copolymer contains 0.01 to 50 mole % of the coumarin derivative, and an average molecular weight of the copolymer is 1,000 to 1,000,000 in terms of polystyrene.
As for a device fabricated in Example 1 of the foregoing Japanese document, it is described that when a 15 V DC voltage was applied to the fabricated device in the atmosphere, then approximately 10 mA/cm2 of electric currents flowed, and uniform and stable white surface-emission with a 300 cd/m2 luminance was obtained. Therefore, a current luminous efficiency at this time is evaluated as approximately 3.0 cd/A. As for a device fabricated in Example 2, it is described that when a 15 V DC voltage was applied to the fabricated device in the atmosphere, then approximately 50 mA/cm2 of electric currents flowed, and uniform and stable white surface-emission with a 400 cd/m2 luminance was obtained. Therefore, a current luminous efficiency at this time is evaluated as approximately 0.8 cd/A.
In addition, the Japanese document discloses in FIG. 1 an emission spectrum of the device of Example 2. In this case, the highest luminescence peak is viewed around a 440 nm wavelength, and the second luminescence peak is viewed around a 560 nm wavelength, as shown in FIG. 1.
However, the above described document makes no description as to CIE chromaticity coordinate values representing specific hues, nor the life or the heat resistivity of light emitting devices. Also, the technology disclosed in the document allows achievement of white emission, but not blue emission in the disclosed organic EL device.
In a method disclosed in the foregoing document, a luminescent layer is formed by spin coating a toluene solution including a copolymer and a fluorescence dye over a substrate at 6000 rpm, and then drying the coated substrate at 50° C. under a 10−1 Pa reduced pressure, thereby forming a thin film constituted by the copolymer and the fluorescence dye.
It is, in general, considered difficult to make a thin film of polymer by vacuum vapor deposition. Thus, there is a disadvantage that in order to form a luminescent layer by making a thin film of polymer, the formation of the thin film must be made by spin coating using such harmful organic solvent as toluene, as described above. Further, the foregoing thin film formation method by spin coating the copolymer has a disadvantage that it is difficult to form a thin film with a uniform thickness over a large substrate.